1. Field of the Invention
The present invention relates to an optical element in which three or more optical elements are cemented together, and an optical system including the optical element, and is suitable for use in an optical system used for an optical apparatus such as a silver-halide film camera, a digital still camera, a video camera, a telescope, binoculars, a projector, or a copying machine.
2. Description of the Related Art
In recent years, an optical system used for an image pickup apparatus such as a digital camera or a video camera (image pickup optical system) is required to have a small total system size and high performance, with a reduction in size and improvement in function of the image pickup apparatus.
In the optical systems generally used for the image pickup apparatus such as the digital camera or the video camera (image pickup optical system), various aberrations increase as a total lens length is shortened and downsizing of the entire optical system is attained.
In particular, chromatic aberrations such as an on-axis chromatic aberration and a lateral chromatic aberration are increasingly caused. As a result, the optical performance significantly reduces. For example, in a case of a telephoto type optical system having a reduced total lens length, the chromatic aberrations increase as a focal length is extended (lengthened), resulting in significantly reducing the optical performance.
Up to now, optical systems using an optical member made of an optical material, for example, a resin which has strong extraordinary partial dispersion have been known as the optical systems having a reduced chromatic aberration (Japanese Patent Application Laid-Open Nos. 2005-181392, 2006-145823, and 2005-352265).
In the inventions disclosed in Japanese Patent Application Laid-Open Nos. 2005-181392, 2006-145823, and 2005-352265, the chromatic aberration of the optical member made of a resin material having strong extraordinary partial dispersion and being imparted with an optical power and the chromatic aberration of the optical member made of another glass material are suitably balanced with each other, to thereby excellently reduce the chromatic aberrations of the entire optical system.
Besides, as optical elements using the optical member made of the resin material, there are known a so-called combined optical element in which a resin layer is formed on a lens surface which has been polished into a spherical shape to provide an aspherical shape, a hybrid aspherical lens, and the like (Japanese Patent Application Laid-Open Nos. H05-008231 and 2001-004920).
In the inventions disclosed in Japanese Patent Application Laid-Open Nos. H05-008231 and 2001-004920, in order to form the aspherical shape on the lens surface, a resin is stacked on a lens which being a base. Then, an aspherical-shaped mold is pressed to the resin to form the lens surface having the aspherical shape.
Further, there are known an optical element in which distortion and peeling of the surface when the multiple optical members are cemented together are reduced, an optical element in which the contact between the resin and the glass is improved, an optical element in which the environmental resistance of the resin is increased, and the like (Japanese Patent Application Laid-Open Nos. 2003-139916 and 2003-147281).
In the method of forming the resin layer on the lens surface using the mold, a degree of difficulty of the formation increases as the resin layer becomes thicker. This is because the absolute amount of curing contraction at the time of formation increases as the amount of the resin increases. As a result, it is difficult to finish the resin surface with high precision. An absolute amount of a change in size of the resin layer to a change in temperature increases as the resin layer becomes thicker, and hence the influence on the optical performance becomes larger.
Japanese Patent Application Laid-Open Nos. 2005-181392, 2006-145823, and 2005-352265 discloses optical systems in which an optical element including a resin layer sandwiched by two lenses is used for chromatic aberration correction.
When the cemented optical element in which the resin layer is sandwiched by the lenses is to be manufactured, the resin layer is formed on a surface of one of the lenses, and then a surface of the resin is covered with a bonding material and the other of the lenses is bonded to the one of the lenses. With this, the influence of precision of the surface of the formed resin layer formed on the optical performance is minimized.
In view of environmental resistance, the cemented optical element has such a feature that deformation is hardly caused, because the surface of the resin layer is regulated by an external lens.
However, when the lenses of the cemented optical element are to be finally cemented to each other, cement distortion or cement peeling under environments may sometimes occur. In the inventions disclosed in Japanese Patent Application Laid-Open Nos. 2003-139916 and 2003-147281, in order to reduce the cement distortion or cement peeling of the lenses, the lenses are bonded to each other using an elastic member as a bonding material.
The bonding method described above is suited to cement two lenses, or to cement a lens and a plastic material to each other. However, an optical element including three optical members, particularly, an optical element manufactured by cementing, after formation of a resin layer on one of two lenses, the two lenses together with another one of the two lenses causes the following problem.
FIG. 14 is a principal cross-sectional view illustrating an optical element including three optical members cemented together. An optical element 14 illustrated in FIG. 14 has a structure in which a resin layer (optical member) NL1 is formed on an optical member L1, and then cemented to an optical member L2.
FIG. 15 is an enlarged schematic view illustrating a part of an outer region of the optical element 14 illustrated in FIG. 14. In view of the degree of difficulty of the formation and the environmental resistance, an absolute thickness of the resin layer NL1 is desirably minimized as much as possible.
Therefore, the thickness of an outer region of the resin layer NL1 along an optical axis 1 is more likely to become thinner than the thickness of a normal polished lens.
Thus, the optical members L1 and L2 are arranged very closely to each other in the outer region. This is remarkable particularly in the case where the resin layer NL1 has a positive power.
The inventors of the present invention found that, when an outer diameter φr of the resin layer NL1 is smaller than a polishing diameter φg of a cemented surface L2b of the optical member L2 as illustrated in FIG. 15 in a state described above, a bonding material S1 used to cement the optical members L1 and L2 together enters between the optical members L1 and L2.
When the bonding material S1 enters between the optical members L1 and L2, the thickness of the bonding material S1 significantly differs between a region having a diameter equal to or smaller than the outer diameter φr and a region having a difference between the diameters φr and φg (band region). Therefore, the absolute amount of curing contraction of the bonding material S1 significantly differs, and hence an optical surface of the optical member L1 or L2 is distorted, to thereby greatly affect the optical performance.
The inventors of the present invention found that the bonding material S1 is more likely to enter between the optical members L1 and L2 in an outer-diameter position of the optical member L1, in which the optical members L1 and L2 are closest to each other, resulting in inducing large distortion.